<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>id选择器</title>
</head>
<style>
  #red {
    color: red;
  }
</style>

<body>
  <div id="red">div 标签</div>
</body>

</html>